It is often necessary, for reasons of safety and security, to know if an object, person, or animal is blocking a defined area. Automatic closure operators such as garage door openers must be able to detect the presence of any such obstruction in the path of the closure so that the motion of the closure can be arrested or reversed immediately. This feature prevents injury to people and animals and additionally prevents damage to property and to the closure operator itself.
There are several methods of sensing obstructions in the path of the closure. One method of performing this function involves utilizing a pressure sensitive strip along the leading edge of the door. In order to sense that an obstruction is in the path of the closure, the strip must first make physical contact with the obstruction with some minimum force. By the time that the pressure registers with the control mechanism and the motion of closure is stopped or reversed, substantial injury or damage could already have been incurred.
Another method of determining whether there is any obstruction in the path of a door is to provide a light source which projects a light beam across the doorway. A light detector is disposed opposite the light source to detect the light beam. When the light detector senses that the beam has been interrupted, the motion of the door is arrested or reversed. In some applications, especially in low power applications, the light beam is pulsed at some predetermined frequency to make it distinguishable from the ambient light conditions. The shortcoming of this method is that most semiconductor light detectors only respond to changes in light intensity within a narrow band. The light detectors have an inherent light intensity threshold below which they can not respond. Additionally, the light detector will saturate if the intensity of light received exceeds the light detector's saturation level. In both circumstances, the light detector would not sense the pulsed light beam from the light source, and the obstruction sensing feature of the closure operator would not function properly. It is toward the solution of this shortcoming that the present invention is directed.